1. Field of the Invention
The present invention relates to a semiconductor laser device, and, more particularly, to a semiconductor laser device having a diffraction grating layer with a phase shift structure used for optical communication.
2. Description of the Related Art
In response to demand for a large-capacity, long-distance information transmission system, development of an optical transceiver has recently been pursued, with a view towards attaining a transmission rate of 10 Gbps.
A distributed feedback semiconductor laser having a diffraction grating provided in the direction of an optical waveguide effects optical feedback corresponding to the period of the diffraction grating, thus enabling single-mode emission. Thus, the distributed feedback semiconductor laser has been developed for optical communication.
Japanese Patent Application Laid-Open No. Hei. 1-155677 describes an invention pertaining to a distributed feedback (DFB) semiconductor laser. In relation to a related-art DFB semiconductor laser, the positional relationship between reflection surfaces provided at both ends of the laser in the direction of an optical waveguide and the phase of the diffraction grating affects an oscillation characteristic having a single longitudinal mode. For this reason, a xcex/4 phase-shifted structure has been employed, and both end surfaces of the laser are covered with a non-reflection coating, thereby minimizing the reflectance of the end faces. If the product of a coupling factor xcexa and a cavity length L, that is, xcexaL, is not in the vicinity of a value of 1.25, an axial hole burning phenomenon arises, and, in turn, deteriorates a single longitudinal mode characteristic of the laser. In order to solve the problem, there is described a DFB semiconductor laser, in which the reflectance of optical power of one end surface is set to 30% and the reflectance of the other end is set to 5 to 15%, thereby achieving a xcexaL value of 0.4xe2x89xa6xcexaLxe2x89xa61.3.
The related-art DFB semiconductor laser induces an axial hole burning phenomenon when the product of a coupling factor xcexa and a cavity length L, that is, xcexaL, is not in the vicinity of a value of 1.25, thus deteriorating a single longitudinal mode characteristic of the laser. A high-precision, non-reflection coating technique has been pursued, and there has been devised a window structure for embedding end-face sections of a waveguide for minimizing the reflectance of the end faces. In light of the above-described drawbacks, Japanese Patent Application Laid-Open No. Hei. 2-20087 describes a DFB semiconductor laser which is comparatively easy to manufacture and has a structure capable of realizing a single longitudinal mode at high yield. The DFB semiconductor laser has one or more phase shift regions within 20% of the resonance length with reference to the center of the cavity. The reflectance of respective end faces is set to 5 to 15%, and the product xcexaL is set to the range of 0.6xe2x89xa6xcexaLxe2x89xa61.0.
In relation to the related-art DFB semiconductor laser, the diffraction grating is adjusted such that the product of a coupling factor xcexa and a cavity length L, that is, xcexaL, assumes a value of 1.2 to 1.3. However, such a semiconductor laser involves a high oscillation threshold current and it tends to saturate in optical output more than other DFB semiconductor lasers under high-temperature operation. To solve the problem, Japanese Patent Application Laid-Open No. Hei. 2-90688 describes a xcex/4 phase-shifted DFB semiconductor laser. In relation to the laser, an optical guide layer is provided between an active layer and a cladding layer, wherein the thickness of the optical guide layer changes at a period which is an integer multiple of half the wavelength of traveling light. Further, the energy gap of the optical guide layer is greater than that of the active layer and smaller than that of the cladding layer. The product of a coupling factor xcexa and a cavity length L, that is, xcexaL, is set to a value of 1.5 to 2.5.
In relation to the related-art DFB semiconductor laser, a multilayer dielectric film is formed on an output end face of the laser, thereby reducing optically-induced return noise. This also drastically reduces optical output from the output end face. In order to reduce the optically-induced return noise and to ensure sufficient optical output, Japanese Patent Application Laid-Open No. Hei. 5-48197 describes a xcex/4 phase-shifted DFB semiconductor laser which is constructed as follows. Namely, the output end face of the laser is covered with a non-reflective coating. Provided that a length from a rear end face to a xcex/4 phase shift point is taken as Ls and the length of a laser cavity is taken as L, xcex/4 phase shift is located in a position where 0.2xe2x89xa6Ls/Lxe2x89xa60.4 is obtained. Further, the product of a coupling factor xcexa and a cavity length L, that is, xcexaL, is set to the range of 2xe2x89xa6xcexaLxe2x89xa64. The laid-open patent publication states that, when measurement was effected through use of an element having a cavity length of 300 xcexcm, superior single-mode oscillation was ascertained to arise even at a xcexaL value of about 3, and good current-light output was obtained.
Japanese Patent Application Laid-Open No. Hei. 6-204607 describes an improvement in the yield and efficiency of a DFB semiconductor laser, including an analog modulation distortion characteristic and a single-mode characteristic. In order to obtain a low-cost, low-distortion analog modulation DFB semiconductor laser, there is employed a structure wherein the reflectance of the front end face of the cavity is less than 5% and wherein the product of a coupling factor xcexa and a cavity length L, that is, xcexaL, is set to the range of 0.4xe2x89xa6xcexaLxe2x89xa61.0.
Further, the related-art xcex/4 phase-shifted DFB semiconductor laser element has encountered difficulty in achieving compatibility of high stability of single mode, a high-yield characteristic, and a high efficiency-and-output characteristic. For this reason, Japanese Patent Application Laid-Open No. Hei. 11-68220 describes a DFB semiconductor laser, wherein a low-reflection film is formed on an optical output end face and wherein a high-reflection film is formed on the other end face. Further, a diffraction grating is formed in a part of the element in the direction toward the cavity. The length of an area where the diffraction grating is to be fabricated is set to 52% to 64% the element length. The product of a coupling factor of the diffraction grating and the length of the diffraction grating fabrication area is set to the range of 0.8 to 2.
On pp. 1261 to 1279 of IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. 25, NO. 6, JUNE 1989, James E. A. Whiteaway et al. describe a xcex/4 phase-shifted DFB semiconductor laser whose active layer does not have the function of a diffraction grating. In relation to the laser, under the conditions where the length L of a diffraction grating area is 50 to 600 xcexcm and a xcexaL value produced from a coupling factor xcexa and the length L of the diffraction grating area falls within a range of 3.0 or less, there result a certain distribution of threshold current (see FIG. 11 on p. 1271), a certain distribution of threshold current density (see FIG. 12 on p. 1271), and a certain distribution of efficiency (see FIG. 13 on p. 1272). According to the figures, the shorter L and the greater the xcexaL value, the smaller the threshold current. Moreover, the longer L and the greater the xcexaL value, the smaller the threshold current density. It is understood that the smaller L and the smaller the xcexaL value, the greater the efficiency.
However, no detailed descriptions are given of high-speed-operation characteristic and the stability of single axial mode characteristic. The report may provide a highly-efficient structure having a low threshold current density but has failed to disclose a structure possessing a sufficient high-speed-operation characteristic and a stable single axial mode characteristic.
On pp. 125 to 126, ECOC 2000 26th European Conference on Optical Communication, Proceedings Volume 1: Monday, Sep. 4, 2000, G. Sakaino et al. reported a phase-shifted DFB semiconductor laser. The laser has an active layer made of InGaASP-based material, a cavity length as short as 200 xcexcm, and a diffraction grating of high xcexaL value. An active layer of the laser does not have any function of a diffraction grating. However, the relaxation oscillation frequency, which is one factor for limiting high-speed operation of a laser, remains at a value of less than 15 GHz (14.9 GHz stated in the paper).
On pp. 89 to 90, 2000 IEEE 17TH International Semiconductor Laser Conference 25 to 28 Sep. 2000 Hyatt Monterey, Monterey Calif., CONFERENCE DIGEST P13, G. Sakaino et al. reported a phase-shifted DFB semiconductor laser. The laser has an active layer made of InGaAsP-based material, a cavity length as short as 200 xcexcm, and a diffraction grating having a high xcexaL value. An active layer of the laser does not have any function of a diffraction grating. In relation to the laser, the relaxation oscillation frequency remains at a value of 12.0 GHz or thereabouts.
If the relaxation oscillation frequency is low, relaxation oscillation cannot be removed even when a receiver employs an electric filter. As a result, the sensitivity of the receiver is deteriorated, thus posing a problem in attaining 10 Gb/s operation.
In order to achieve a transmission rate of 10 Gb/s, a relaxation oscillation frequency of 30 GHz or more is desired. However, it is empirically seen that, if a relaxation oscillation frequency of 15 GHz or more is not obtained, a sufficient eye-pattern opening will not be obtained, thus inducing non_negligible deterioration in receiving sensitivity.
Moreover, IEEE PHOTONICS TECHNOLOGY LEFITERS, VOL. 7, NO. 10, OCTOBER 1995, PP. 1119 to 1121 reports a multiple reflection short-length cavity of active layer isolation type having a xcex/4 phase-shifted structure.
Appl. Phys. Lett. 57(6), Aug. 6, 1990, pp. 534 to 536 reports a xcex/4 phase-shifted DFB semiconductor laser having a xcexaL value of 9.
IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. 27, NO. 6, JUNE 1991 pp. 1753-1758 describes a xcex/4 phase-shifted DFB semiconductor laser having a device length of 172 xcexcm, a coupling factor xcexa of 330 cmxe2x88x921, and a xcexaL value of 5.6.
The present invention has been made to overcome the above-described drawbacks and disadvantages of the related art. It is an object of the present invention to provide a semiconductor laser having a low threshold current density/high efficiency characteristic, a sufficient high-speed-operation characteristic, and a stable single axial mode characteristic.
According to one aspect of the invention, there is provided a semiconductor laser device comprising: a semiconductor substrate of first conductivity type; a first cladding layer of first conductivity type provided on the semiconductor substrate; an active layer provided on the first cladding layer and having uniformly flat upper and lower boundary surfaces in the direction of an optical waveguide; a second cladding layer of second conductivity type provided on the active layer, and a diffraction grating layer having a phase-shifted structure, provided in the direction of the optical waveguide between the first cladding layer and the active layer or between the second cladding layer and the active layer, wherein the length L of the diffraction grating layer in the direction of the optical waveguide is taken as Lxe2x89xa6260 xcexcm; a mean coupling factor xcexa of a diffraction grating layer is taken as xcexaxe2x89xa7150 cmxe2x88x921; and a value xcexaL, which is the product of the length L and the mean coupling factor xcexa, is taken as 5.6 greater than xcexaL greater than 3.0, whereby the present invention can achieve a semiconductor laser device having a high relaxation oscillation frequency fr, a stable single axial mode, and sufficient slope efficiency.
Accordingly, the present invention enables configuration of a low-cost, highly-reliable semiconductor laser having a superior high-speed characteristic. Hence, there can be constructed a low-cost transmission system of 10 Gb/s or more.
Other objects and advantages of the invention will become apparent from the detailed description given hereinafter. It should be understood, however, that the detailed description and specific embodiments are given by way of illustration only since various changes and modifications within the scope of the invention will become apparent to those skilled in the art from this detailed description.